This invention relates generally to message store-and-forward communication systems and particularly to time division multiple access (TDMA) communication systems, such as satellite-linked systems, which further operate with more than a single channel group.
Though not limited thereto, the invention is particularly useful as an improvement of a communication system in which a central station referred to as a "land earth station" (LES) or a "hub" communicates via satellite with a number of "mobile terminals" over a forward channel of a designated channel group, and the mobile terminals communicate with the hub over respective return channels of the channel group. The term "mobile terminal" is used herein to refer to one of a group of individual terminals that are communicating with the hub. The term "mobile terminal", as used herein, refers to the characteristic of terminals being subject to physical movement within a reception region of the system.
State of the art communication systems, as those referred to herein, generally operate over satellite links. Communication satellites covering, for example, North America relay messages over a number of area-specific spot beams which, in total, may cover the entire Continent. In such environment, the communcation system may operate in any particular spot beam with one primary time-division multiplex (TDM) forward channel, and one or more secondary TDM forward channels, wherein each TDM forward channel may carry the forward channel communication to a portion of all mobile terminals logged into the system. Each TDM channel operates with a designated number of associated return channels to make up a distinct channel group in such a multiple channel-group system.
In a specific example with respect to which the invention is described, the central station or LES operates a communication system which serves a number of individual customers, for example a number of trucking companies, each of which may operate and require control over hundreds or even in excess of a thousand trucks. The mobile terminals are installed on respective trucks of the customer trucking companies, for "star type", point-to-multipoint, communications between respective truck dispatch centers and enroute trucks of each of the trucking companies.
Satellite communication links are a limited and costly commodity. There is always a need to improve efficiency of communications over satellite communication links to lower the cost of communication systems using such satellite links. For cost reasons, the channel groups of the system are typically limited in number to match the network with anticipated load levels, although, allowing some safety margin. However, owing to variations in message traffic levels, the network can get overloaded through exhaustion of available channel capacity, or even through operational failure of the LES caused by temporary peak overload conditions.
In general, prior art LES operations seek to balance communication loads by distributing login assignments of existing mobile terminals substantially evenly over available channel groups controlled by the LES. Primary login frequencies, one per spot beam, are pre-programmed into each of the mobile terminals. The primary login frequencies are assigned and preprogrammed into each mobile terminal when it is placed into service.
Message traffic loads do not always correspond to the number of terminals assigned to operate over particular channel groups of the network. A truck may require little or no communication while on one route, and may require a substantial amount of communication while on another route. Congestion over a channel group may be caused by a single, relatively long message.
According to current practice, an LES may reserve, on demand, special "LES TDMs" or special forward channels for communicating such longer messages to the particular recipient mobile terminal, requiring such recipient mobile terminal, which is already logged in on its primary TDM channel frequency, to change its frequency the LES TDM for the receipt of the message, and, subsequently, to return to the preassigned primary TDM forward channel. While the use of on-demand, or demand-assigned, TDM channels can reduce forward traffic loads on the primary TDM channel, it does not mitigate traffic in the return direction, which is carried by a fixed set of return channels. Further, requiring the mobile terminals to change the frequency to and from the "LES TDM" adds to message transport delays and is, for that reason, undesirable.
Also, according to current practices, when a particular channel group in a spot beam becomes overloaded, an operator can prevent additional mobile terminals from logging into the channel group by setting a flag or bit in a broadcast bulletin board which acts as a busy signal. Mobile terminals, which are assigned the particular, busy channel group as a primary channel group, will be prevented from logging into the prior art communication system as long as the flag remains set. The prior art terminals are programmed to periodically check whether the busy flag has been reset, and if so, to log in at such time.
An LES, pursuant to other known practices for avoiding temporary, potentially harmful network overload conditions, may also assign a mobile terminal to a secondary channel group when the mobile terminal attempts to first log into a preassigned primary TDM channel. Assignments of mobile terminals at the time of login are made to maintain a distribution of channel loads when several channel groups are available whithin any given spot beam area. The assignments do not affect mobile terminals which are already communicating over the channel group of the primary TDM channel. An assignment of a mobile terminal to a secondary TDM channel and its associated return channels is permanent for the duration of the login period of the mobile terminal. Reassignment of a mobile terminal on initial login, as desirable as it may seem, has been found not to be able to cope whith unexpected traffic growth from the mobile terminals already logged into a channel group. Moreover, a procedure, which simply blocks further mobile terminals from logging into a particular channel group, fails to address the essence of the problem in that such procedure does not support the attainment of a substantially uniform distribution of traffic across a number of available channel groups.
Risks of communication problems due to imbalances of traffic loads among available channel groups continues to exist in current systems. There is a continued need to arrive at more economical procedures for balancing channel loads and to avoid sudden, temporary traffic congestion and overload conditions in certain channel groups and possible system failures as a result thereof.